1. Field of the Invention
The present invention relates to a digital radio communication receiver for deciding synchronous words used for decision of received frame synchronization and decision of received frame structure in accordance with correlative thresholds different from each other.
2. Description of the Prior Art
Generally, in the case of digital radio communication, a received bit series is detected by detecting a received signal and received information is correctly extracted by detecting the frame timing in the received bit series. Detection of frame timing and frame synchronization are performed by detecting a bit series having a sharp autocorrelation set to a predetermined position in a frame, that is, by detecting a synchronous word. The synchronous word is also referred to as Sync word or Unique word. Moreover, in figures, a synchronous word is shown as UW that is the abbreviation of Unique word.
A synchronous word is detected by comparing a received bit series with a synchronous word bit series prepared at the reception side. When the number of mismatched bits of the received bit series and the synchronous word bit series is equal to or less than a correlative threshold serving as a predetermined threshold, it is decided that a synchronous word is detected. However, when the number of mismatched bits exceeds the correlative threshold at the timing at which a synchronous word should be originally present, it is decided that synchronous word is false detected.
When frame synchronization is established, it is possible to approximately estimate the position of a synchronous word in a receiver. Therefore, when frame synchronization is established, the false detection probability of synchronous words can be lowered by setting a gate referred to as aperture and detecting an effective synchronous word at the position of the synchronous word or only in a small range around the position.
Frame synchronization is established by detecting synchronous words at a predetermined position continuously by a specified number of frames. This operation is referred to as backward protection and the specified number of frames is referred to as the number of backward protection stages. As the number of stages increases, false frame synchronization does not easily occur and therefore, the reliability of frame synchronization is improved. However, it takes a lot of time until frame synchronization is established. However, when the number of stages decreases, it takes only a short time until frame synchronization is established. However, false frame synchronization easily occurs.
Frame step-out is detected when synchronous words are missed continuously by a specified number of frames at a position at which a synchronous word should be originally present. This operation is referred to as forward protection and the specified number of frames is referred to as the number of forward protection stages. As the number of stages increases, it takes a lot of time to detect frame step-out due to deterioration of circuit quality. Conversely, as the number of stages decreases, it takes only a short time to detect frame step-out. However, the probability of erroneously deciding frame step-out is raised when frame synchronization should be originally held.
Moreover, in the case of a digital radio communication system, frame structures may be changed depending on the communication state. For example, in the case of a system using the voice actuation art, a frame is transmitted only when a voice is significant but no frame is transmitted in principle when no voice is recognized. Even in this case, however, a short burst including a synchronous word is transmitted every certain interval in order to hold frame synchronization. However, the interval is generally different from a frame length.
As described above, when frame structures are changed depending on the communication state, it is necessary that the transmission side communicates the change of frame structures. To communicate the change of frame structures, there is a method of previously setting a bit series for communicating a frame structure in a frame and moreover, there is a method of inserting a bit series for communicating change of frame structures (hereafter referred to as frame structure flag). Moreover, at the reception side, there are a method of detecting a frame structure flag and a method of deciding a frame. structure every synchronous-word detection interval.
FIG. 4 is a block diagram showing the structure of a section for estimating frame synchronization and frame structure of the digital radio communication receiver disclosed in the Japanese Patent Laid-Open publication No. 247114/1997. In FIG. 4, reference numeral 1xe2x80x2 denotes a synchronous word detecting section, 2 denotes an aperture control section, 3 denotes a timing control section, 4 denotes a number-of-frame-synchronization protection stage setting section, 5 denotes a frame synchronization deciding section, 6 denotes an aperture width setting section, 7xe2x80x2 denotes a correlative threshold setting section, 8 denotes a received signal extracting section, 9 denotes a frame structure deciding section, 100 denotes an antenna, 101 denotes a down converter, and 102 denotes a wave detector.
Then, operations of the conventional example in FIG. 4 are described below. The down converter 101 converts a carrier wave received from the antenna 100 into an intermediate frequency band and the wave detector 102 demodulates a signal received from the intermediate frequency band to output it as a received bit series. The synchronous word detecting section 1xe2x80x2 receiving the received bit string takes the correlation between received bit series and synchronous words in accordance with the timing information sent from the aperture control section 2, performs detection of a synchronous word and decision of a phase in accordance with the number of error bits and the correlative threshold sent from the correlative threshold setting section 7, and outputs the decision results to the timing control section 3, frame synchronization deciding section 5, frame structure deciding section 9, aperture width setting section 6, and correlative threshold setting section 7 as synchronous word detecting information. The timing control section 3 outputs the received timing formation used to next perform synchronous word detection to the aperture control section 2 and received signal extracting section 8 in accordance with the synchronous word detecting information. The frame synchronization deciding section 5 decides a frame synchronous state using the number of forward protection stages serving as the continuous detection frequency of the synchronous word detecting information designated by the number of frame-synchronization protection stages 4 and the number of backward protection stages serving as the number of continuous misses, and outputs the decision result to the aperture width setting section 6 and correlative threshold setting section 7 as frame synchronization deciding information. The aperture width setting section 6 sets an aperture width serving as a time width used to next perform synchronous word detection by using the synchronous word detecting information and frame synchronization information and outputs the aperture width to the aperture control section 2. The correlative threshold setting section 7 sets a correlative threshold serving as a synchronous word detecting condition used to next perform synchronous word detection by using the synchronous word detecting information and frame synchronization information and outputs the correlative threshold to the synchronous word detecting section 1. The received signal extracting section 8 extracts a received signal from a received bit series in accordance with received timing information. The frame structure deciding section 9 decides a frame structure in accordance with the detection interval of the synchronous word detecting information and outputs the decision result as frame structure deciding information.
Then, recognition of a frame structure for performing frame synchronization control by the above digital radio communication receiver is described below by referring to FIG. 5. FIG. 5 shows a state in which frame structures are changed depending on the communication state. A part of FIG. 5 is shown by extracting FIG. 3 in xe2x80x9cRADIO TRANSMISSION IN THE AMERICAN MOBILE SATELLITE SYSTEMxe2x80x9d, A COLLECTION OF TECHNICAL PAPERS,AIAA-94-0945-CP, pp. 280-294 (1994) and simplifying FIG. 3.
In FIG. 5, reference numeral 23 denotes a synchronous word. 24 denotes a first frame structure flag showing a first frame structure, which is inserted when a second frame structure changes to the first frame structure. 25 denotes a second frame structure flag showing a second frame structure, which is inserted when the first frame structure changes to the second frame structure. The frame structure includes the following two types: the first frame structure having a synchronous word every subframe and the second frame structure having a synchronous word every frame. Moreover, the frame structure deciding section 9 shown in FIG. 4 decides the first frame structure when detecting a synchronous word at a specified interval in which the first frame structure flag 24 or the first frame structure can be recognized and decides the second frame structure when detecting a synchronous word at a specified interval in which the second frame structure flag 24 or the second frame structure can be recognized.
FIGS. 6 and 7 are illustrations showing examples of recognizing the frame structure in FIG. 4. FIG. 6 shows a case in which the received first frame structure is falsely detected as the second frame structure. FIG. 7 shows a case in which the received second frame structure is falsely detected as the first frame structure. In this case, it is assumed that the first frame structure serving as a continuous frame and the second frame structure serving as a channel activity burst for transmitting a synchronous word every four frames are present. Moreover, when a synchronous word is detected for three consecutive frames, it is decided that the first frame structure is recognized. When a synchronous word is detected only at a one-frame interval, it is decided that the second frame structure is recognized.
FIG. 8 is a chart showing the relation between correlative thresholds used for synchronous word decision results and probabilities of missed and falsely-detected frame structure decision results so as to examine the probability causing the erroneous decisions shown in FIGS. 6 and 7. In the case of FIG. 8, when assuming that the received frame synchronization state is synchronous state and the value of a correlative threshold is 6, the probability of missed synchronous words is 1.49xc3x9710xe2x88x922 and the probability of falsely-detected synchronous words is 2.68xc3x9710xe2x88x924. When a received frame structure is the second frame structure, the probability of not deciding the second frame structure as the second frame structure which cannot be detected is 3.06xc3x9710xe2x88x922 and the probability of falsely deciding the second frame structure as the first frame structure is 2.12xc3x9710xe2x88x927. When a received frame structure is the first frame structure, the probability not capable of detecting the first frame structure is 4.40xc3x9710xe2x88x922 and the probability of falsely deciding the second frame structure as the first frame structure is 3.26xc3x9710xe2x88x926.
FIG. 9 is an illustration obtained by graphing the values in FIG. 8. From FIG. 9, it is found that using a correlative threshold for minimizing the probability of missed frame structures is more proper for frame structure decision because the probabilities of falsely-detected frame structures shown by E and G are considerably low compared to the probabilities of missed frame structures shown by D and F. Moreover, by considering the first and second frame structures, it is estimated that a correlative threshold of 7 to 8 is proper. However, because a correlative threshold is used to decide frame synchronization in the case of the conventional example, it is impossible to obtain a correlative threshold of 7 to 8 that is suitable for a frame structure.
A conventional digital radio communication receiver is constituted as described above and the synchronous word detecting information detected by the synchronous word detecting section 1xe2x80x2 is directly used for the frame synchronization deciding section 5 and frame structure deciding section 9. Therefore, there is a problem that it is impossible to optimize a frame synchronization probability and frame structure decision probability.
The present invention is made to solve the above problem and its object is to provide a reception method and a receiver of a digital radio communication system, capable of improving a frame synchronization probability and a frame structure decision probability.
According to one aspect of the invention, a reception method for a digital radio communication system includes the steps of directly detecting a frame-synchronization synchronous word from a received bit series, directly detecting a frame-structure synchronous word from the received bit series, deciding a frame synchronous state in accordance with the frame-synchronization frame synchronous word, deciding a frame structure in accordance with the frame-structure synchronous word, and outputting each decision result.
According to another aspect of the invention, a reception method for a digital radio communication system includes the steps of separately detecting a plurality of different types of frame-structure synchronous words, directly from the same received bit series of a digital radio communication signal, deciding a plurality of different types of frame-structures in accordance with the frame-structure synchronous words, and outputting each decision result.
According to yet another aspect of the claimed invention, a receiver for a digital radio communication system having two types of frame structures is provided, wherein a frame-synchronization synchronous word is decided from a received bit series, a first frame structure synchronous word is decided from the same received bit series, first and second correlative thresholds are set for determining the frame synchronization and frame structure decisions, and frame synchronization and frame-structure deciding circuits detect received frames and frame-structures in accordance with the frame-synchronization and frame-structure synchronous word detection information.